Electroluminescent element and light-emitting device

ABSTRACT

An electroluminescent element which can easily control the balance of color in white emission (white balance) is provided according to the present invention. The electroluminescent element comprises a first light-emitting layer containing one kind or two or more kinds of light-emitting materials, and a second light-emitting layer containing two kinds of light-emitting materials (a host material and a phosphorescent material) in which the phosphorescent material is doped at a concentration of from 10 to 40 wt %, preferably, from 12.5 to 20 wt %. Consequently, blue emission can be obtained from the first light-emitting layer and green and red (or orange) emission can be obtained from the second light-emitting layer. An electroluminescent element having such device configuration can easily control white balance since emission peak intensity changes at the same rate in case of increasing a current density.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electroluminescent elementcomprising an anode, a cathode, and a layer including organic compounds(hereinafter, an electroluminescent layer) which emit light by applyingcurrent through the pair of electrodes; and a light-emitting devicewhich comprises the electroluminescent layer. More specifically, thisinvention relates to an electroluminescent element which exhibits whitelight emission, and a full color light-emitting device comprising theelectroluminescent element.

[0003] 2. Related Art

[0004] An electroluminescent element comprises an electroluminescentlayer interposed between a pair of electrodes (anode and cathode). Theemission mechanism is as follows. Upon applying voltage through the pairof electrodes, holes injected from an anode and electrons injected froma cathode are recombined with each other within the electroluminescentlayer to result in the formation of molecular excitons, and themolecular excitons return to the ground state while radiating energy toemit photon. There are two excited states possible from organiccompounds, a singlet state and a triplet state. It is considered thatlight emission is possible through both the singlet state and thetriplet state.

[0005] Although an electroluminescent layer may have a single layerstructure comprising only a light-emitting layer formed by alight-emitting material, the electroluminescent layer is formed to havenot only a single layer structure comprising only a light-emitting layerbut also a lamination layer structure having a hole injecting layer, ahole transporting layer, a hole blocking layer, an electron transportinglayer, an electron injecting layer, and the like which are formed by aplurality of functional materials.

[0006] It is known that color tone can be appropriately changed bydoping tiny amounts of fluorescent substances (typically, at mostapproximately 10³ mol % based on the value of host substances) into hostsubstances within the light-emitting layer. (For example, refer toJapanese Patent Publication No. 2,814,435.)

[0007] Besides, the following are known as methods for changing colortone. Blue light emission obtained from a light-emitting layer is usedas a light emission source, and the obtained emission color is convertedinto desired color within a color changing layer formed by colorchanging materials (hereinafter, CCM method). Alternatively, white lightemission obtained from a light-emitting layer is used as a lightemission source, and the obtained emission color is converted intodesired color by a color filter (hereinafter, CF method).

[0008] However, in case of adopting the CCM method, there has been aproblem in red color since color conversion efficiency of from blue tored is poor in principle. In addition, there has been a problem that thecontrast becomes deteriorated from light emission in pixels due tooutside light such as sunlight since color conversion materials arefluorescent materials. Therefore, it is considered that CF methodwithout such problems is preferably used.

[0009] In the case of using CF method, an electroluminescent elementexhibiting white light emission (hereinafter, white light emissionelement) having high luminance is required since much light is absorbedin a color filter.

[0010] With respect to the white light emission element, elements formedby various materials to have various configurations have been reported.It is quite important to control the balance of emission color (whitebalance) since white light emission is obtained by a plurality ofmaterials, each of which emits different color, meanwhile it isdifficult to do that.

[0011] For example, in the case that white light emission is obtained bymixing a plurality of materials, each of which exhibits emission inblue, green and red colors, it has been reported that an emission peakintensity of each the materials is different depending on currentdensity. (For example, refer to Brien W. D'Andrede, Jason Brooks, VadimAdamovich, Mark E. Thompson, and Stephen R. Forrest, Advanced Material(2002), 14, No.15, August 5, 1032-1036 (FIG. 2)) In case of forming suchelement, peak intensity of each emission color changes in differentrates. Consequently, it becomes extremely difficult to control whitebalance which is adjusted by the parameter which determines the peakintensity of these emission colors.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide anelectroluminescent element which can be easily controlled the colorbalance in white light emission (white balance).

[0013] As the result of research for solving the above mentionedproblems, the inventor found that the rate of changes of peak intensitybecomes hardly changed depending on light-emitting materials even when acurrent density is increased by keeping the concentration oflight-emitting materials among a plurality of light-emitting materialsused for obtaining white light emission in a certain range.

[0014] In view of the foregoing, an electroluminescent layer in anelectroluminescent element comprises a first light-emitting layercontaining one kind or two or more kinds of light-emitting materials;and a second light-emitting layer containing two kinds of light-emittingmaterials in which one of them is doped at a concentration of from 10 to40 wt %, preferably, from 12.5 to 20 wt %.

[0015] In the above described structure, blue emission having anemission peak intensity in the wavelength region of from 400 to 500 nmcan be obtained from the first light-emitting layer by using one kind ortwo or more kinds of light-emitting materials; and green emission havingan emission peak intensity in the wavelength region of from 500 to 550nm and red (or orange) emission having an emission peak intensity in thewavelength region of from 550 to 700 nm can be obtained from the secondlight-emitting layer by using two kinds of light-emitting materials,host materials and phosphorescent materials, to dope phosphorescentmaterials at a concentration of from 10 to 40 wt %, preferably, 12.5 to20 wt %. However, as the phosphorescent materials, materials which canresult in the formation of an excited dimer (excimer) formed by the bondof excited atoms or molecules with ground state atoms or molecules areused here.

[0016] As stated above, the first light-emitting layer exhibiting blueemission and the second light-emitting layer exhibiting green and red(or orange) emission are formed separately. In consequence, there is anadvantage that the electroluminescent element becomes easy tomanufacture since the control of the concentration or the like forforming the first light-emitting layer becomes facilitated. Further,compared with the case that all color emission is obtained from onelight-emitting layer, it can be expected that the varies of emissionwavelength, the decrease of peak intensity, or the like due to theinteraction of molecules having different structures from each other(exciplex). within a light-emitting layer is prevented.

[0017] Further, by doping phosphorescent materials into the secondlight-emitting layer within the above described concentration ranges,not only the number of excimer formed from phosphorescent materials canbe controlled, but also the light emission (green emission and redemission) can be obtained from the second light-emitting layersimultaneously with blue emission from the first light-emitting layer.In this case, the peak intensity of phosphorescent emission (greenemission) obtained from phosphorescent materials and emission (red (ororange)) from excimer (the peak intensity of both the emission can beconsidered to be the same because the intensity ratio is depending onthe concentration) and the peak intensity of blue emission from thefirst light-emitting layer changes at the almost same rate in case ofincreasing current density, hence, the peak intensity is easy to controland white light emission with well white balanced can be easilyobtained.

[0018] Therefore, one of constituent features of the invention is thatan electroluminescent element comprises an electroluminescent layerinterposed between a pair of electrodes wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of from 500 to 700 nm; and the second light-emitting layercontains a phosphorescent material which forms excimer at aconcentration of from 10 to 40 wt %, preferably, from 12.5 to 20 wt %.

[0019] Another constituent features of the invention is that anelectroluminescent element comprises an electroluminescent layerinterposed between a pair of electrodes wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of from 500 to 700 nm; and the second light-emitting layercontains a phosphorescent material which forms excimer at aconcentration of at least 10⁴ mol/cm³ and at most 10³ mol/cm³.

[0020] By setting a concentration of the phosphorescent material withinthe above described ranges, an emission peak intensity of green lightemission (occurred in a wavelength region of from 500 to 550 nm) to red(or orange) light emission (occurred in a wavelength region of from 550to 700 nm) has a ratio of from 50 to 150%, preferably, from 70 to 130%.

[0021] Additionally, by setting a concentration of the phosphorescentmaterial within the above described ranges, a luminance of from 100 to2000 cd/m², preferably, from 300 to 1000 cd/m² is obtained from theelectroluminescent element.

[0022] More additionally, by setting a concentration of thephosphorescent material within the above described ranges, a part of thephosphorescent material can be existed at a certain distance from oneanother so that the molecules of the phosphorescent material can formexcimer emission.

[0023] Besides, by setting a concentration of the phosphorescentmaterial within the above described ranges, the phosphorescent materialformed by a metal complex can be existed so that central metals of thephosphorescent materials are a distance of from 2 to 20 Å from oneanother.

[0024] In the above each constituent features, the second light-emittinglayer is formed to have preferably the thickness of from 20 to 50 nm,more preferably, from 25 to 40 nm.

[0025] Further, in the above each constituent features, the firstlight-emitting layer has an emission spectrum with an emission peak in awavelength region of from 400 to 500 nm; the second light-emitting layerhas an emission spectrum with an emission peak in a wavelength region offrom 500 to 700 nm; and any one of the plurality of emission peaks isexcimer emission.

[0026] In the above each constituent features, the phosphorescentmaterial is organic metal complex with platinum as a central metal.

[0027] The invention comprehends an electric appliance and alight-emitting device, each of which comprises the above describedelectroluminescent element.

[0028] Accordingly, an electroluminescent element can be provided whichcan be controlled easily the color balance in white light emission(white balance) by keeping the concentration so as to be within acertain range of light-emitting materials among a plurality oflight-emitting materials used for obtaining white light emission.

[0029] These and other objects, features and advantages of the inventionwill become more apparent upon reading of the following detaileddescription along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIGS. 1A to 1C are views showing light emission mechanism of anelectroluminescent element according to the present invention;

[0031]FIGS. 2A and 2B are band diagrams showing a device configurationof an electroluminescent element according to the invention;

[0032]FIGS. 3A and 3B are band diagrams showing a device configurationof an electroluminescent element according to the invention;

[0033]FIGS. 4A and 4B are band diagrams showing a device configurationof an electroluminescent element according to the invention;

[0034]FIG. 5 is a view showing a specific device configuration of anelectroluminescent element according to the invention;

[0035]FIGS. 6A and 6B are schematic views of a light-emitting deviceaccording to the invention;

[0036]FIGS. 7A to 7G are examples of electric appliances using alight-emitting device according to the invention;

[0037]FIG. 8 shows an emission spectrum according to Example 2 andComparative Example 1;

[0038]FIG. 9 shows current density dependence of an emission spectrumaccording to Example 2;

[0039]FIG. 10 shows luminance-current characteristics according toExample 2 and Comparative Example 1;

[0040]FIG. 11 shows luminance-voltage characteristics according toExample 2 and Comparative Example 1;

[0041]FIG. 12 shows current efficiency-current characteristics accordingto Example 2 and Comparative Example 1; and

[0042]FIG. 13 shows current-voltage characteristics according to Example2 and Comparative Example 1.

DESCRIPTION OF THE INVENTION

[0043] An electroluminescent element according to the present inventioncomprises a pair of electrodes (an anode and a cathode) and anelectroluminescent layer having at least a first light-emitting and asecond light-emitting layer interposed between the pair of electrodes.As shown in FIG. 1A, within the first light-emitting layer,light-emitting materials are excited by recombination of carriers, andmonomers in excited states are formed, then, light-emission is obtained(blue emission: hv₁). Within the second light-emitting layer,phosphorescent materials are excited by recombination of carriers, andmonomers in excited states are formed, then, phosphorescent emission isobtained (green emission: hv₂). Simultaneously, within the secondlight-emitting layer, monomer in the excited state and monomer in theground state form an excited dimer (excimer), and excimer emission (red(or orange): hv₂′) can also be obtained.

[0044] Within the second light-emitting layer, since an energy state ofthe excimer state obtained from phosphorescent materials is lower thanthat of the excited state obtained from phosphorescent materials asshown in FIG. 1B, the excimer emission (hv₂′) is always at longerwavelength side than the general phosphorescent light emission (hv₂)(specifically, at least several ten nm longer wavelength side).Therefore, in the case that phosphorescent materials which can generatephosphorescent emission at a green emission wavelength region are usedas in the invention, excimer emission is at the red emission wavelengthregion. Hence, according to the invention, by combining green emissionand red emission from phosphorescent materials with blue emission fromanother light-emitting materials, high efficient white light emissionhaving peak intensity in each red, green, and blue wavelength region canbe obtained.

[0045] For forming an excimer state from phosphorescent materials, it isrequired to make it easier for monomer in an excited state and monomerin a ground state to form an excited dimer by their interaction.Specifically, phosphorescent materials are. preferably doped into hostmaterials at concentration of from 10 wt % to 40 wt %, more preferably,from 12.5 wt % to 20 wt % within the second light-emitting layer.Besides, phosphorescent materials having high planarity structures suchas platinum complex are preferably used as guest materials to keep thedistance between central ions (or atoms) of the phosphorescent materialswithin a certain range. According to the invention, in the case that thephosphorescent materials (monomer) in a ground state and phosphorescentmaterials (monomer) in an excimer state are respectively at the positiondenoted by (a) and the position denoted by (b) as shown in FIG. 1C, thedistance between central ions (d₁) is preferably from 2 to 5 Å. However,phosphorescent materials (monomer) in a ground state can interact withphosphorescent materials (monomer) in an excited state even when thephosphorescent materials (monomer) in a ground state are at the positiondenoted by (c). Further, in consideration that the average radius (r) ina molecular structure of phosphorescent materials according to theinvention is approximately from 6 to 9 Å, the distance between centralions (d₂) is preferably 2 Å<d₂<20Å.

[0046] The first light-emitting layer exhibiting blue emission can beformed by a single substance (blue luminous body), or host materials andguest materials (blue luminous body).

[0047] For forming an electroluminescent element according to theinvention, a device design is required so that both the firstlight-emitting layer and the second light-emitting layer to emit light.Specifically, the relationship of ionization potential among the firstlight-emitting layer, the second light-emitting layer, and anotherlayers, each of which composes an electroluminescent layer is requiredto be most appropriate.

[0048] In addition, the device design becomes different depending on theconfiguration of functional layers composing an electroluminescentlayer, subsequently, preferred embodiment of the invention will bedescribed in terms of the relationship between a device configurationand a band diagram hereinafter.

Embodiment 1

[0049] In Embodiment 1, as shown in FIG. 2A, the case that a firstelectrode 201, an electroluminescent layer 202, and a second electrode203 are formed over a substrate 200, and that the electroluminescentlayer 202 has a lamination structure comprising a first light-emittinglayer 211, a second light-emitting layer 212, and an electrontransporting layer 213 will be explained. In addition, the firstlight-emitting layer 211 includes a light-emitting body. The secondlight-emitting layer 212 comprises host materials 251 and phosphorescentmaterials 252 which serve as a light-emitting body. The phosphorescentmaterials can generate both phosphorescent emission and excimeremission.

[0050] As a light-emitting body (light-emitting material) for the firstlight-emitting layer 211, blue fluorescent materials having holetransportation properties such asN,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(abbreviated TPD) or derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereafter, referred toas α-NPD); or blue fluorescent materials having electron transportationproperties such asbis(2-methyl-8-quinolinolate)-(4-hydroxy-biphenylyl)-aluminum(abbreviated BAlq) or bis [2-(2-hydroxyphenyl)-benzooxazolate] zinc(abbreviated Zn(BOX)₂). Various blue fluorescent dyes, for example,perylene, 9,10-diphenyl anthracene, or coumarin based fluorescent dyes(coumarin 30 or the like) can be used as guest materials. Further,phosphorescent materials such asbis(4,6-difluorophenyl)pyridinato-N,C²′) (acetylacetonato)iridium(abbreviated lr(Fppy)₂(acac)) can be used. All of these materials haveemission peak intensity in the wavelength region of from 400 to 500 nm,so that they are suitable for materials for the light-emitting body ofthe first light-emitting layer 211 according to the invention.

[0051] An organic metal complex with platinum as a central metal isefficiently used for a light-emitting body (phosphorescent material) ofthe second light-emitting layer 212. Specifically, by doping substancesrepresented by the structural formulas 1 to 4 at concentration of from10 wt % to 40 wt %, preferably, from 12.5 wt % to 20 wt % into hostmaterials, both phosphorescent emission and its excimer emission can beobtained. However, the present invention is not limited thereto, anyphosphorescent material can be used as long as both phosphorescentemission and excimer emission can be simultaneously obtained therefrom.

[0052] In case of using guest materials for the first light-emittinglayer and the second light-emitting layer comprising a light-emittingbody, hole transportation materials or electron transportation materialstypified by the following examples can be used. In addition, bipolarmaterials such as 4,4′-N,N′-dicarbazolyl-biphenyl (abbreviated CBP) canbe used as host materials.

[0053] As hole transportation materials, aromatic amine (that is, theone having a benzene ring-nitrogen bond) compounds are preferably used.For example,N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(abbreviated TPD) or derivatives thereof such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated α-NPD) iswidely used. Also used are star burst aromatic amine compounds,including: 4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine (abbreviatedTDATA); and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine. (abbreviated MTDATA).

[0054] As electron transportation materials, metal complexes such astris(8-quinolinolate) aluminum. (abbreviated Alq₃),tnrs(4-methyl-8-quinolinolate) aluminum (abbreviated Almq₃),bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviated BeBq₂), BAlq,Zn(BOX)₂, and bis [2-(2-hydroxyphenyl)-benzothiazolate] zinc(abbreviated Zn(BTZ)₂). Additionally, oxadiazole derivatives, such as2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviatedPBD), and 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl] benzene(abbreviated OXD-7); triazole derivatives such as3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated TAZ) and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenyly)-1,2,4-triazole(abbreviated p-EtTAZ); imidazol derivatives such as2,2′,2″-(1,3,5-benzenetryil)tris 1-phenyl-1H-benzimidazolel (abbreviatedTPBI); and phenanthroline derivatives such as, bathophenanthroline(abbreviated BPhen) and bathocuproin (abbreviated BCP) can be used inaddition to metal complexes.

[0055] In addition, the above described electron transporting materialscan be used for the electron transporting layer 213.

[0056]FIG. 2B is a band diagram in case of forming a device having theabove described configuration. The diagram shows a HOMO level(ionization potential) 220 of the first light-emitting layer 201; a HOMOlevel (ionization potential) 221 and a LUMO level 222 of the firstlight-emitting layer 211; a HOMO level (ionization potential) 223 and aLUMO level 224 of host materials of the second light-emitting layer 212in a thin film shape; a HOMO level (ionization potential) 225 and a LUMOlevel 226 of guest materials (phosphorescent materials) of the secondlight-emitting layer 212; a HOMO level (ionization potential) 227 and aLUMO level 228 of the electron transporting layer 213; and a LUMO level229 of the second electrode 203, respectively.

[0057] In this instance, an energy gap 230 between the ionizationpotential 221 of the first light-emitting layer 211 and the ionizationpotential (in this instance, the ionization potential of the hostmaterials 251 is considered that of the whole second light-emittinglayer 212) 223 of the whole second light-emitting layer 212 (in thestate that both the host materials 251 and the phosphorescent materials252 are included) is preferably sufficiently large (specifically, atleast 0.4 eV). If the energy gap 230 is small, holes inject to thesecond light-emitting layer 212 from the first light-emitting layer 211since the first light-emitting layer 211 has hole transportationproperties. Consequently, the majority of carriers are recombined withinthe second light-emitting layer 212. Therefore, since the secondlight-emitting layer 212 exhibits emission of the wavelength region ofgreen to red, energy cannot transfer to the first light-emitting layer211 which exhibits blue emission of further shorter wavelength.Accordingly, only the second light-emitting layer emits light.

[0058] By making the energy gap 230 sufficiently large, the majority ofcarriers are recombined with each other at the vicinity of an interfacebetween the first light-emitting layer 211 and the second light-emittinglayer 212. The other small number of carries are recombined within thesecond light-emitting layer 212, or trapped partly in the HOMO level 214of the phosphorescent materials, consequently, both the light-emittinglayer 211 and the second light-emitting layer 212 can emit light.

[0059] The same is true in the case that guest materials generating blueemission are contained in the host materials within the firstlight-emitting layer 211. That is, the energy gap 230 between theionization potential 221 of the whole light-emitting layer 211 (in thestate that both host materials of the first light-emitting layer and theguest materials generating blue emission are included) and theionization potential 223 of the whole second light-emitting layer 212(in the state that both the host materials 251 of the secondlight-emitting layer and the phosphorescent materials 252 are included)is preferably large (specifically, at least 0.4 eV).

[0060] Further, an energy gap 231 between the ionization potential 223of the whole second light-emitting layer 212 (in the state that both thehost materials 221 and the phosphorescent materials 222 are included)and the ionization potential 227 of the electron transporting layer 213is preferably large (specifically, at least 0.4 eV). In this instance,by making the energy gap 231 large, holes serving as carriers can betrapped in the second light-emitting layer 212, hence, carriers can beefficiently recombined within the second light-emitting layer 212.

[0061] Moreover, an energy gap 232 between the LUMO level 222 of thefirst light-emitting layer 211 and the LUMO level (in this instance, theLUMO level of the host materials is considered that of the whole secondlight-emitting layer) 224 of the whole second light-emitting layer 212(in the state that both the host materials 251 and the phosphorescentmaterials 252 are included) is preferably large (specifically, at least0.4 eV). In this instance, by making the energy gap 232 large, holesserving as carriers can be trapped in the second light-emitting layer212, hence, recombination can be took place efficiently in the secondlight-emitting layer 212.

[0062] Therefore, light emission from the first light-emitting layer 211and the second light-emitting layer 212 can be further efficientlyobtained by a band-gap structure having energy gaps 230, 231, and 232 inEmbodiment 1.

Embodiment 2

[0063] In Embodiment 2, the case that a first electrode 301, anelectroluminescent layer 302, and a second electrode 303 are formed overa substrate 300, and the electroluminescent layer 302 has a laminationstructure comprising a first light-emitting layer 311 and a secondlight-emitting layer 312 will be explained with reference to FIG. 3A. Inaddition, second light-emitting layer 312 comprises host materials 351and phosphorescent materials 352. The phosphorescent materials cangenerate phosphorescent emission and excimer emission.

[0064] Embodiment 2 is distinguished from Embodiment 1 by the fact thatthe electroluminescent layer does not comprise the electron transportinglayer, and so Embodiment 2 has an advantage that the process for formingthe electron transporting layer can be omitted. In addition, materialshaving excellent electron transportation properties are preferably usedas host materials 321 for the second light-emitting layer 312 in orderto keep luminous efficiency.

[0065] In Embodiment 2, materials for forming the first light-emittinglayer 311 and the second light-emitting layer 312 are the same as thosedescribed in Embodiment 1.

[0066]FIG. 3B is a band diagram in case of forming a device having theabove described configuration. The diagram shows a HOMO level(ionization potential) 320 of the first light-emitting layer 301; a HOMOlevel (ionization potential) 321 and a LUMO level 322 of the firstlight-emitting layer 311; a HOMO level (ionization potential) 323 and aLUMO level 324 of host materials of the second light-emitting layer 312in a thin film shape; a HOMO level (ionization potential) 325 and a LUMOlevel 326 of guest materials (phosphorescent materials) of the secondlight-emitting layer 312; and a LUMO level 327 of the second electrode303, respectively.

[0067] In Embodiment 2 just as Embodiment 1, in order to obtainefficiently light from the first light-emitting layer 311 and the secondlight-emitting layer 312, an energy. gap 341 between the ionizationpotential 321 of the first light-emitting layer 311 and the ionizationpotential (in this instance, the ionization potential of the hostmaterials 351 is considered that of the whole second light-emittinglayer 312) 323 of the whole second light-emitting layer 312 (in thestate that both the host materials 351 and the phosphorescent materials352 are included) is preferably sufficiently large (specifically, atleast 0.4 eV). Further, an energy gap 342 between the LUMO level 322 ofthe first light-emitting layer 311 and the LUMO level (in this instance,the LUMO level of the host materials 351 is considered that of the wholesecond light-emitting layer 312) 324 of the whole second light-emittinglayer 312 (in the state that both the host materials 351 and thephosphorescent materials 352 are included) is preferably sufficientlylarge (specifically, at least 0.3 eV).

[0068] The same is true in the case that guest materials generating blueemission are contained in host materials within the first light-emittinglayer 311. That is, the energy gap 341 between the ionization potential(in this instance, the ionization potential of the host materials of thefirst light-emitting layer 311 is considered the ionization potential ofthe whole first light-emitting layer 311) 321 of the wholelight-emitting layer 311 (in the state that both the host materials andthe guest materials generating blue emission are included) and theionization potential 323 of the whole second light-emitting layer 312(in the state that both the host materials 351 and the phosphorescentmaterials 352 are included) is preferably large (specifically, at least0.4 eV).

Embodiment 3

[0069] In Embodiment 3, the case that a first electrode 401, anelectroluminescent layer 402, and a second electrode 403 are formed overa substrate 400, and that the electroluminescent layer 402 has alamination structure comprising a hole injecting layer 411, a firstlight-emitting layer 412, and a second light-emitting layer 413, and anelectron transporting layer 414 will be explained with reference to FIG.4A. In addition, second light-emitting layer 413 comprises hostmaterials 451 and phosphorescent materials 452 that serve as alight-emitting body. The phosphorescent materials can generatephosphorescent emission and excimer emission.

[0070] As materials for the hole injecting layer 411, besides the abovedescribed hole transporting materials such as TPD, α-NPD, TDATA, orMTDATA, the following hole transporting materials can be used.

[0071] As hole injection materials, porphyrin compounds are useful amongother organic compounds such as phthalocyanine (abbreviated H₂-PC),copper phthalocyanine (abbreviated Cu—Pc), or the like. Further,chemical-doped conductive polymer compounds can be used, such aspolyethylene dioxythipophene (abbreviated PEDOT) doped with polystyrenesulfonate (abbreviated PSS), polyaniline, or polyvinyl carbazole(abbreviated PVK). A thin film of an inorganic semiconductor such asvanadium pentoxide or an ultra thin film of an inorganic insulator suchas aluminum oxide can also be used.

[0072] As a light-emitting body or materials for the firstlight-emitting layer 412, a second light-emitting layer 413, and anelectron transporting layer 414, the same materials described inEmbodiment 1 can be used, respectively.

[0073]FIG. 4B is a band diagram in case of forming a device having theabove described configuration. The diagram shows a HOMO level(ionization potential) 420 of the first electrode 401; a HOMO level(ionization potential) 421 and a LUMO level 422 of the hole injectinglayer 411; a HOMO level (ionization potential) 423 and a LUMO level 424of the first light-emitting layer 412; a HOMO level (ionizationpotential) 425 and a LUMO level 426 of host materials of the secondlight-emitting layer 413; a HOMO level (ionization potential) 427 and aLUMO level 428 of guest materials (phosphorescent materials) of thesecond light-emitting layer 413; a IOMO level (ionization potential) 429and a LUMO level 430 of the electron transporting layer 414; and a LUMOlevel 431 of the second electrode 403, respectively.

[0074] The configuration described in Embodiment 3 is distinguished fromthat described in Embodiment 1 by the fact that the hole transportinglayer 411 is included.

[0075] In Embodiment 3, in order to obtain light further efficientlyfrom the first light-emitting layer 412, and the second light-emittinglayer 413, an energy gap 441 between the ionization potential 423 of thefirst light-emitting layer 412 and the ionization potential (in thisinstance, the ionization potential of the host materials 451 isconsidered that of the whole second light-emitting layer 413) 425 of thewhole second light-emitting layer 413 (in the state that both the hostmaterials 451 and the phosphorescent material 452 are included) ispreferably sufficiently large (specifically, at least 0.4 eV). Likewise,an energy gap 442 between the ionization potential 425 of the wholesecond light-emitting layer 413 (in the state that both the hostmaterials 451 and: the phosphorescent material 452 are included) and theionization potential 429 of the electron transporting layer 414 ispreferably sufficiently large (specifically, at least 0.4 eV).Additionally, an energy gap 443 between the LUMO level 424 of the firstlight-emitting layer 412 and the LUMO level (in this instance, the LUMOlevel of the host material 451 is considered that of the whole secondlight-emitting layer 413) 426 of the whole second light-emitting layer413 (in the state that both the host materials 451 and thephosphorescent materials 452) is preferably sufficiently large(specifically, at least 0.3 eV). Moreover, an energy gap 444 between theLUMO level 422 of the hole injecting layer 411 and the LUMO level 423 ofthe first light-emitting layer 412 is preferably sufficiently large(specifically, at least 0.3 eV).

[0076] By holding the energy gap 444 between the LUMO level 422 of thehole injecting layer 411 and the LUMO level 423 of the firstlight-emitting layer 412, electrons can be trapped into the firstlight-emitting layer 412, and so carriers can be efficiently recombinedwith each other in the first light-emitting layer 412.

[0077] The same is true in the case that guest materials generating blueemission are contained in host materials within the first light-emittinglayer 412. That is, an energy gap 441 between the ionization potential(in this instance, the ionization potential of host materials of thefirst light-emitting layer is considered that of the whole firstlight-emitting layer 412) 423 of the whole first light-emitting layer412 (in the state that both the host materials and the guest materialsgenerating blue emission are included) and the ionization potential 425of the whole second light-emitting layer 413 (in the state that both thehost materials 451 and the phosphorescent materials 452 are included) ispreferably sufficiently large (specifically, at least 0.4 eV).

[0078] Accordingly, by applying the typical configurations described inEmbodiments 1 to 3 according to the present invention, a whiteelectroluminescent element having peak intensity in each wavelengthregion of red, green, and blue can be achieved with such a simple deviceconfiguration.

[0079] Further, the above described configuration is illustrative onlyas one of preferred configurations. An electroluminescent layer in aelectroluminescent element according to the invention may comprise atleast the above described first light-emitting layer and secondlight-emitting layer. Therefore, though not nominated in thisspecification, a layer having properties except light-emissionproperties (for example, an electron injection layer, or the like) whichis known as used in the conventional electroluminescent element can beappropriately used.

[0080] As electron injection materials for forming an electron injectinglayer, above described electron transportation materials can be used.Additionally, a ultra thin film of insulator, for example, alkalinemetal halogenated compounds such as UF or CsF; alkaline earthhalogenated compounds such as CaF₂; or alkaline metal oxides such asLi₂O is often used. Further, alkaline metal complexes such as lithiumacetylacetonate (abbreviated Li(acac)), 8-quinolinolato-lithium(abbreviated Liq) can also be used.

[0081] An electroluminescent element according to the invention may beformed in such a way that at least either electrode is formed bytransparent materials in order to extract light through either theelectrode. Generally, the configuration that a first electrode formedover a substrate is transparent to light (also referred to as a bottomemission structure); the configuration that a second electrode which isstacked over an electroluminescent layer formed over the first electrodeis transparent to light (also referred to as a top emission structure);or the configuration that both electrodes are transparent to light (alsoreferred to as a dual emission structure) may be adopted.

[0082] As anode materials for either the first electrodes (201, 301, and401) or the second electrodes (203, 303, and 403), conductive materialshaving large work functions are preferably used. When light is extractedthrough an anode, transparent conductive materials such as indium-tinoxides (ITO) or indium-zinc oxides (IZO) may be used. When an anode isformed to have light shielding properties, a single layer formed by TiN,ZrN, Ti, W, Ni, Pt, Cr, or the like; a lamination layer comprising thesingle layer and a film containing titanium nitride and aluminum as itsmain components; a three lamination layer comprising a titanium nitridefilm, a film containing aluminum as its main components, and a titaniumnitride film; or the like can be used. Alternatively, the anode can beformed by stacking the above described transparent conductive materialsover a reflective electrode of Ti, Al, or the like.

[0083] As materials for a cathode, conductive materials having smallwork functions are preferably used. Specifically, alkaline metals suchas Li or Cs; alkaline earth metals such as Mg, Ca, or Sr; alloys oftheses metals (Mg: Ag, Al: Li, or the like); or rare earth metals suchas Yb or Er can be used. In addition, in case of using an electroninjecting layer formed by LiF, CsF, CaF₂, Li₂O, or the like, theconventional conductive thin film such as aluminum can be used. In caseof extracting light through cathode, the cathode can be formed to have alamination structure comprising a ultra thin film containing alkalinemetals such as Li or Cs, or alkaline earth metals such as Mg, Ca or Srand a transparent conductive film (ITO, IZO, ZnO, or the like).Alternatively, the cathode may be formed by forming an electroninjecting layer by alkaline metals or alkaline earth metals, formingelectron transportation materials by co-evaporation, and stacking atransparent conductive-film (ITO, IZO, ZnO, or the like) thereon.

[0084] In manufacturing the above described electroluminescent elementaccording to the invention, a method for stacking each layer of theelectroluminescent element is not limited. Any method of vacuum vapordeposition, spin coating, ink jetting, dip coating, or the like can beused, as long as layers can be stacked by these methods.

EXAMPLE 1

[0085] Hereinafter, examples of the present invention will be explained.

[0086] In this example, a device configuration of an electroluminescentelement and a method for manufacturing thereof according to theinvention will be explained with reference to FIG. 5.

[0087] An anode 501 of the electroluminescent element was formed over aglass substrate 500 having an insulating surface. As a material for theanode 501, ITO, a transparent conductive film, is used. The anode 501 isformed by sputtering to have a thickness of 110 nm. The anode 501 issquare in shape and 2 mm in height and width.

[0088] Then, an electroluminescent layer 502 is formed over the anode501. In this example, the electroluminescent layer 502 has a laminationstructure comprising a hole injecting layer 511; a first light-emittinglayer 512 which has hole injection properties; a second light-emittinglayer 513; an electron transporting layer 514; and an electron injectinglayer 515. The first light-emitting layer 512 is formed by materialswhich can achieve blue emission, specifically, materials which has anemission spectrum with maximum intensity in the wavelength region offrom 400 to 500 nm. In addition, the second light-emitting layer 513 isformed by host materials or guest materials which generatephosphorescent light emission.

[0089] First, a substrate provided with the anode 501 is secured with asubstrate holder of a vacuum deposition system in such a way that thesurface provided with the anode 501 is down. Then, Cu—Pc is put into anevaporation source installed in the internal of the vacuum depositionsystem. And then, the hole injection layer 511 is formed to have athickness of 20 nm by vacuum vapor deposition with a resistive heatingmethod.

[0090] Then, the first light-emitting layer 512 is formed by a materialwhich has excellent hole transportation properties and light-emissionproperties. In this example, α-NPD is deposited in accordance with thesame procedures as those conducted for forming the hole injection layer511 to have a thickness of 30 nm.

[0091] And then, the second light-emitting layer 513 is formed. In thisexample, the second light-emitting layer 513 is formed by CBP as hostmaterials and Pt(ppy)acac represented by the structural formula 1 asguest materials which are controlled to be 15 wt % in concentration tohave a thickness of 20 nm by co-evaporation.

[0092] Further, the electron transporting layer 514 is formed over thesecond light-emitting layer 513. The electron transporting layer 514 isformed by BCP (bathocuproin) to have a thickness of 20 nm by vapordeposition. CaF₂ is deposited to have a thickness of 2 nm as theelectron injection layer 515 thereon to complete the electroluminescentlayer 502 having a lamination structure.

[0093] Lastly, a cathode 503 is formed. In this example, the cathode 503is formed by aluminum (Al) by vapor deposition with a resistive heatingmethod to have a thickness of 100 nm.

[0094] Therefore, an electroluminescent element according to theinvention is formed. In addition, in the device configuration describedin Example 1, each the first light-emitting layer 512 and the secondlight-emitting layer 513 can exhibit light emission, so that a devicethat exhibits white light emission as a whole can be formed.

[0095] In this example, an anode is formed over a substrate; however,the invention is not limited thereto. A cathode can be formed over asubstrate. In this case, that is, in case of exchanging an anode tocathode, lamination sequence of the electroluminescent layer describedin this example is reversed.

[0096] In this example, the anode 501 is a transparent electrode inorder to extract light generated in the electroluminescent layer 502from the anode 501; however, the invention is not limited thereto. Ifthe cathode 503 is formed by a selected material that is suitable forsecuring transmittance, light can be extracted from the cathode.

EXAMPLE 2

[0097] In this example, device characteristics of the electroluminescentelement described in Example 1 having the configuration: ITO/Cu—Pc (20nm)/α-NPD (30 nm)/CBP+Pt(i,py)acac: 15 wt % (20 nm)/BCP (30 nm)/CaF (2nm)/Al (100 nm) will be explained. Emission spectrum of theelectroluminescent element having the above described configuration isshown by spectrum 1 in FIG. 8, and FIG. 9. Each plot 1 in FIGS. 10 to 13shows for electric characteristics.

[0098] Spectrum 1 in FIG. 8 shows the emission spectrum of theelectroluminescent element having the above described configuration atan applied current of 1 mA (at a luminance of approximately 960 cd/m²).From the result shown by spectrum 1, white light emission can beobtained having three components: blue emission from α-NPD composing thefirst light-emitting layer (around 450 nm); green emission fromphosphorescent light emission of Pt(ppy)acac contained in a secondlight-emitting layer (around 490 nm, around 530 nm); and orange emissionfrom excimer emission of Pt(ppy)acac contained in the secondlight-emitting layer. CIE chromaticity coordinate was. (x, y)=(0.346,0.397). The light emission was almost white in appearance.

[0099] Ionization potential of the α-NPD used for the firstlight-emitting layer and the CBP used for the second light-emittinglayer was measured. The α-NPD had ionization potential of approximately5.3 eV, and the CBP had that of approximately 5.9 eV The difference inthe ionization potential between the α-NPD and the CBP was approximately0.6 eV. Therefore, preferable condition of the invention, that is,ionization potential of at least 0.4 eV, was satisfied. Consequently, itcan be considered that the fact brought about good white light emission.In addition, the measurement of ionization potential was carried outwith photoelectron spectrometer (AC-2) (RIKEN KEIKI Co., Ltd.).

[0100]FIG. 9 shows measurement results of each spectrum at differentamount of current flow in the electroluminescent element having theabove described configuration. FIG. 9 shows measurement results atdifferent amount of current flow denoted by spectrum a (0.1 mA),spectrum b (1 mA), and spectrum c (5 mA). Clearly from the measurementresults, a spectral shape was hardly changed even when the amount ofcurrent flow was increased (luminance was increased). It can beconsidered that the electroluminescent element according to theinvention exhibits stable white light emission, which is hardly affectedby the change of the amount of current flow.

[0101] As electric characteristics of the electroluminescent elementhaving the above described configuration, the luminance-current plot 1in FIG. 10 shows that a luminance of approximately 460 cd/m² wasobtained at a current density of 10 mA/cm².

[0102] The luminance-voltage plot 1 in FIG. 11 shows that a luminance ofapproximately 120 cd/m² was obtained at an applied voltage of 9 V.

[0103] The current efficiency-luminance plot 1 in FIG. 12 shows thatcurrent efficiency of approximately 4.6 cd/A was obtained at a luminanceof 100 cd/m².

[0104] The current-voltage plot 1 in FIG. 13 shows that a current flowwas approximately 0.12 mA at an applied voltage of 9 V.

[0105] The obtained amount of Pt in the above describedelectroluminescent element was 21 ng by quantitative determination byInductively Coupled Plasma-Mass Spectrometry (ICP-MS). The obtainedatomic concentration per unit area was 5.4×10¹⁴ atoms/cm² by convertingthe amount of Pt.

[0106] Further, depth profiling of Pt concentration was conducted bySecondary Ion Mass Spectrometry (SIMS), and the above described amountof Pt was converted into the original amount, then, calculated theconcentration of Pt per unit volume. In consequence, the maximum valueof Pt concentration per unit was approximately 2.0×10²⁰ atoms/cm³, and3.3×10⁻⁴ mol/cm³ at mol concentration. Therefore, it can be consideredthat excimer emission becomes possible if the concentration ofphosphorescent materials forming excimer is in the range of from 10⁻⁴ to10⁻³ mol/cm³.

[0107] As mentioned above, because of the fact that the maximum value ofPt concentration per unit volume is approximately 2.0×10²⁰ atoms/cm³,the average volume of one Pt complex is 5.0×10⁻²⁷ m³/atom. In case thatthe Pt complex is dispersed evenly, the Pt complex is dispersed inphosphorescent materials in the proportion of one Pt complex to 1.7cubic nm by volume. Therefore, the distance between metal atoms eachother of phosphorescent materials (in this example, Pt atom) isapproximately 17 Å. Hence, the distance between central metals eachother of phosphorescent materials is preferably at most 20 Å accordingto the invention.

COMPARATIVE EXAMPLE 1

[0108] Correspondingly, each spectrum 2 and spectrum 3 in FHG 8 showsemission spectrum measured from an electroluminescent element in which alight-emitting layer comprise Pt(ppy)acac at different concentrationfrom that described in Example 1. The spectrum 2 shows a measurementresult in the case that concentration of Pt(ppy)acac is 7.9 wt %. Thespectrum 3 shows a measurement result in the case that concentration ofPt(ppy)acac is 2.5 wt %. In each case, the spectrum was obtained at thecurrent flow of 1 mA.

[0109] As shown by spectrum 3, in case that Pt(ppy)acac is contained atconcentration of 2.5 wt %, blue emission from α-NPD composing a firstlight-emitting layer (around 450 nm) and green emission from Pt(ppy)acaccontained in a second light-emitting layer (around 490 nm, around 530nm) were only observed, and white light emission has not resulted. Asshown in spectrum 2, in case that Pt(ppy)acac is contained atconcentration of 7.9 wt %, a slight of excimer emission was in thespectrum as a shoulder at the vicinity of 560 nm; however the peak wasinsufficient, consequently, excellent white light emission could not beobserved.

[0110] Further, current characteristics were measured from the devices.Each plot 2 in FIGS. 10 to 13 shows measurement results from the devicecontaining Pt(ppy)acac at concentration of 7.9 wt %. Each plot 3 inFIGS. 10 to 13 shows measurement results from the device containingPt(ppy)acac at concentration of 2.5 wt %.

[0111] The luminance-voltage characteristics in FIG. 10 show that aluminance of approximately 180 cd/m² was obtained from the devicecontaining Pt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 115 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at a current density of 10mA/cm², respectively.

[0112] The luminance-voltage characteristics in FIG. 11 show that aluminance of approximately 93 cd/m² was obtained form the devicecontaining Pt(ppy)acac at concentration of 7.9 wt % and a luminance ofapproximately 73 cd/m² was obtained from the device containingPt(ppy)acac at concentration of 2.5 wt % at an applied voltage of 9 V,respectively.

[0113] The current efficiency-luminance characteristics in FIG. 12 showthat a current efficiency of approximately 1.8 cd/A was obtained fromthe device containing Pt(ppy)acac at concentration of 7.9 wt % and acurrent efficiency of approximately 1.1 cd/A was obtained from thedevice containing Pt(ppyjacac at concentration of 2.5 wt % at theluminance of 100 cd/m², respectively.

[0114] The current-voltage characteristics in FIG. 13 show that acurrent flow was approximately 0.21 mA in the device containingPt(ppy)acac at concentration of 7.9 wt % and a current flow wasapproximately 0.27 mA in the device containing Pt(ppy)acac atconcentration of 2.5 wt % at an applied voltage of 9 V, respectively.The above measurement results (especially, from the result of thecurrent-voltage characteristics shown in FIG. 13) provide the fact thatthe electroluminescent element according to the invention containingPt(ppy)acac as guest materials in high concentration (15 wt %) has thesame level of electric characteristics as those of theelectroluminescent element containing Pt(ppy)acac as guest materials insuch low concentration (7.9 wt %, 2.5 wt %).

EXAMPLE 3

[0115] In this example, an example for manufacturing a light-emittingdevice (top emission structure) having an electroluminescent elementaccording to the present invention which exhibits white light emissionover a substrate having an insulating surface will be explained withreference to FIG. 6. As used herein, the term “top emission structure”refers to a structure that light is extracted from the opposite side ofa substrate having an insulating surface.

[0116]FIG. 6A is a top view of a light-emitting device. FIG. 6B is across-sectional view of FIG. 6A taken along the line A-A′. Referencenumeral 601 indicated by a dotted line denotes a source signal linedriver circuit; 602, a pixel portion; 603, a gate signal line drivercircuit; 604, a transparent sealing substrate; 605, a first sealingagent; and 607, a second sealing agent. The inside surrounded by thefirst sealing agent 605 is filled with the transparent second sealingagent 607. In addition, the first sealing agent 605 contains a gap agentfor spacing between substrates.

[0117] Reference 608 denotes a connecting wiring for transmittingsignals inputted to the source signal line driver circuit 601 and thegate signal line driver circuit 603. The wiring receives video signalsor clock signals from an FPC (flexible printed circuit) 609 serving asan external input terminal. Although only FPC is illustrated in thedrawing, a PWB (printed wirings board) may be attached to the FPC.

[0118] Then, a cross-sectional structure will be explained withreference to FIG. 6B. A driver circuit and a pixel portion are formedover a substrate 610. In FIG. 6B, the source signal line driver circuit601 and the pixel portion 602 are illustrated as a driver circuit.

[0119] The source signal line driver circuit 601 is provided with a CMOScircuit formed by combining an n-channel TFT 623 and a p-channel TFT624. A IFF for forming a driver circuit may be formed by a known CMOS,PMOS, or NMOS circuit. In this example, a driver integrated type, thatis, a driver circuit is formed over a substrate, is described, but notexclusively, the driver circuit can be formed outside instead of over asubstrate. In addition, the structure of a TFT using a polysilicon filmas an active layer is not especially limited. A top gate TFT or a bottomgate TFT can be adopted.

[0120] The pixel portion 602 is composed of a plurality of pixelsincluding a switching TFT 611, a current control TFT 612, and a firstelectrode (anode) 613 connected to the drain of the current control TFT612. The current control TFT 612 may be either an n-channel TFF or ap-channel TFT. In case that the current control TFT 612 is connected toan anode, the TFT is preferably a p-channel TFT. In FIG. 6B, across-sectional structure of only one of thousands of pixels isillustrated to show an example that two TFTs are used for the pixel.However, three or more numbers of TFTs can be appropriately used.

[0121] Since the first electrode (anode) 613 is directly in contact withthe drain of a TFT, a bottom layer of the first electrode (anode) 613 ispreferably formed by a material capable of making an ohmic contact withthe drain formed by silicon, and a top layer which is in contact with alayer containing an organic compound is preferably formed. by a materialhaving large work functions. In case of forming the first electrode(anode) by three layers structure comprising a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, the first electrode (anode) can reduce resistance as a wiring,make a favorable ohmic contact, and function as an anode. Further, thefirst electrode (anode) 613 can be formed by a single layer such as atitanium nitride film, a chromium film, a tungsten film, a zinc film, ora platinum film; or a lamination layer composed of three or more layers.

[0122] Insulator (also referred to as a bank) 614 is formed at the edgeof the first electrode (anode) 613. The insulator 614 may be formed byan organic resin film or an insulating film containing silicon. In thisexample, an insulator is formed by a positive type photosensitiveacrylic film as the insulator 614 in the shape as illustrated in FIG. 6.

[0123] In order to make favorable coverage, an upper edge portion or alower edge portion of the insulator 614 is formed to have a curved facehaving a radius of curvature. For example, in case that positive typephotosensitive acrylic is used as a material for the insulator 614, onlyupper edge portion of the insulator 614 is preferably having a radius ofcurvature (from 0.2 to 3 μm). As the insulator 614, either a negativetype photosensitive resin that becomes insoluble to etchant by light ora positive type photosensitive resin that becomes dissoluble to etchantby light can be used.

[0124] Further, the insulator 614 may be covered by a protective filmformed by an aluminum nitride film, an aluminum nitride oxide film, athin film containing carbon as its main component, or a silicon nitridefilm.

[0125] An electroluminescent layer 615 is selectively formed over thefirst electrode (anode) 613 by vapor deposition. Moreover, a secondelectrode (cathode) 616 is formed over the electroluminescent layer 615.As the cathode, a material having a small work function (Al, Ag, Li, Ca;or alloys of these elements such as Mg: Ag, Mg: In, or Al: Li; or CaN)can be used.

[0126] In order to pass light, the second electrode (cathode) 616 isformed by a lamination layer of a thin metal film having small workfunctions and a transparent conductive film (ITO, IZO, ZnO, or thelike). An electroluminescent element 618 is thus formed comprising thefirst electrode (anode) 613, the electroluminescent layer 615, and thesecond electrode (cathode) 616.

[0127] In this example, the electroluminescent layer 615 is formed by alamination structure explained in Example 1. That is, theelectroluminescent layer 615 is formed by stacking sequentially Cu—Pc asa hole injecting layer (20 nm), α-NPD as a first light-emitting layerhaving hole transporting properties (30 nm), CBP+Pt(ppy)acac:15 wt % (20nm) as a second light-emitting layer, and BCP as an electrontransporting layer (30 nm). In addition, an electron injecting layer(CaF₂) is unnecessary in the device since the second electrode (cathode)is formed by a thin film metal film having small work functions.

[0128] Thus formed electroluminescent element 618 exhibits white lightemission. In addition, a color filter comprising a coloring layer 631and a light shielding layer (BM) 632 is provided to realize full color(for simplification, an over coat layer is not illustrated).

[0129] In order to seal the electroluminescent element 618, atransparent protective lamination layer 617 is formed. The transparentprotective lamination layer 617 comprises a first inorganic insulatingfilm, a stress relaxation film, and a second inorganic insulating film.As the first inorganic insulating film and the second inorganicinsulating film, a silicon nitride film, a silicon oxide film, a siliconoxynitride film (composition ratio: N<O), a silicon nitride oxide film(composition ratio: N>O), or a thin film containing carbon as its maincomponent (for example, a DLC film or a CN film) can be used. Theseinorganic insulating films have high blocking properties againstmoisture. However, when the film thickness is increased, film stress isalso increased, consequently, film peeling is easily occurred.

[0130] By interposing a stress relaxation film between the firstinorganic insulating film and the second inorganic insulating film,moisture can be absorbed and stress can be relaxed. Even when fine holes(such as pin holes) are existed on the first inorganic insulating filmduring forming the film for any reason, the stress relaxation film canfill the fine holes. The second inorganic insulating film formed overthe stress relaxation film gives the transparent protective laminationfilm excellent blocking properties against moisture or oxygen.

[0131] Materials having smaller stress than that of an inorganicinsulating film and hygroscopic properties are preferably used for thestress relaxation film. In addition, a material that is transparent tolight is preferable. As the stress relaxation film, a film containing anorganic compound such as α-NPD, BCP, MTDATA, or Alq₃ can be used. Thesefilms have hygroscopic properties and are almost transparent in case ofhaving thin film thickness. Further, MgO, SrO₂, or SrO can be used asthe stress relaxation film since they have hygroscopic properties andtranslucency, and can be formed into a thin film by vapor deposition.

[0132] In this example, a silicon nitride film which is formed by vapordeposition using a silicon target in the atmosphere containing nitrogenand argon to have high blocking properties against impurities such asmoisture or alkaline metals is used as the first inorganic insulatingfilm or the second inorganic insulating film. Alq3 is deposited to forma thin film as the stress relaxation film by vapor deposition. In orderto pass light through the transparent protective lamination layer, thetotal film thickness of the transparent protective lamination layer ispreferably formed to be thin as possible.

[0133] In order to seal the electroluminescent element 618, the sealingsubstrate 604 is pasted with the first sealing agent 605 and the secondsealing agent 607 in an inert gas atmosphere. Epoxy resin is preferablyused for the first sealing agent 605 and the second sealing agent 607.It is desirable that the first sealing agent 605 and the second sealingagent 607 inhibit moisture or oxygen as possible.

[0134] In this example, as a material for the sealing substrate 604, aplastic substrate formed by FRP (Fiberglass-Reinforced Plastics), PVF(polyvinyl fluoride), Myler, polyester, acrylic, or the like can be usedbesides a glass substrate or a quartz substrate. After pasting thesealing substrate 604 with the first sealing agent 605 and the secondsealing agent 607, a third sealing agent can be provided to seal theside face (exposed face).

[0135] By encapsulating the electroluminescent element 618 in the firstsealing agent 605 and the second sealing agent 607, theelectroluminescent element 618 can be shielded completely from outsideto prevent moisture or oxygen that brings deterioration of theelectroluminescent layer 615 from penetrating into theelectroluminescent element 618. Therefore, a high reliablelight-emitting device can be obtained.

[0136] If a transparent conductive film is used as the first electrode(anode) 613, a dual emission device can be manufactured.

[0137] The light-emitting device according to this example can bepracticed by utilizing not only the device configuration of theelectroluminescent device explained in Example 1 but also combining theconfiguration of the electroluminescent device formed according to theinvention with that explained in Example 1.

EXAMPLE 4

[0138] Various electric appliances completed by using a light-emittingdevice having an electroluminescent element according to the presentinvention will be explained in this example.

[0139] Given as examples of such electric appliances manufactured byusing the light-emitting device having the electroluminescent elementaccording to the invention: a video camera, a digital camera, agoggles-type display (head mount display), a navigation system, a soundreproduction device (a car audio equipment, an audio set and the like),a laptop personal computer, a game machine, a portable informationterminal (a mobile computer, a cellular phone, a portable game machine,an electronic book, or the like), an image reproduction device includinga recording medium (more specifically, a device which can reproduce arecording medium such as a digital versatile disc (DVD) and so forth,and includes a display for displaying the reproduced image), or thelike. FIGS. 7A to 7G show various specific examples of such electricappliances.

[0140]FIG. 7A illustrates a display device which includes a frame 7101,a support table 7102, a display portion 7103, a speaker portion 7104, avideo input terminal 7105, or the like. The light-emitting device usingthe electroluminescent element according to the invention can be usedfor the display portion 7103. The display device is including all of thedisplay devices for displaying information, such as a personal computer,a receiver of TV broadcasting, and an advertising display.

[0141]FIG. 7B illustrates a laptop computer which includes a main body7201, a casing 7202, a display portion 7203, a keyboard 7204, anexternal connection port 7205, a pointing mouse 7206, or the like. Thelight-emitting device using the electroluminescent element according tothe invention can be used to the display portion 7203.

[0142]FIG. 7C illustrates a mobile computer which includes a main body7301, a display portion 7302, a switch 7303, an operation key 7304, aninfrared port 7305, or the like. The light-emitting device using theelectroluminescent element according to the invention can be used to thedisplay portion 7302.

[0143]FIG. 7D illustrates an image reproduction device including arecording medium (more specifically, a DVD reproduction device), whichincludes a main body 7401, a casing 7402, a display portion A 7403,another display portion B 7404, a recording medium (DVD or the like)reading portion 7405, an operation key 7406, a speaker portion 7407 orthe like. The display portion A 7403 is used mainly for displaying imageinformation, while the display portion B 7404 is used mainly fordisplaying character information. The light-emitting device using theelectroluminescent element according to the invention can be used to thedisplay potion A 7403 and the display portion B 7404. Note that theimage reproduction device including a recording medium further includesa domestic game machine or the like.

[0144]FIG. 7E illustrates a goggle type display (head mounted display),which includes a main body 7501, a display portion 7502, and an armportion 7503. The light-emitting device using the electroluminescentelement according to the invention can be used to the display portion7502.

[0145]FIG. 7F illustrates a video camera which includes a main body7601, a display, portion 7602, an casing 7603, an external connectingport 7604, a remote control receiving portion 7605, an image receivingportion 7606, a battery 7607, a sound input portion 7608, an operationkey 7609, an eyepiece potion 7610 or the like. The light-emitting deviceusing the electroluminescent element according to the invention can beused to the display portion 7602.

[0146]FIG. 7G illustrates a cellular phone which includes a main body7701, a casing 7702, a display portion 7703, a sound input portion 7704,a sound output portion 7705, an operation key 7706, an externalconnecting port 7707, an antenna 7708, or the like. The light-emittingdevice using the electroluminescent element according to the inventioncan be used to the display portion 7703. Note that the display portion7703 can reduce power consumption of the cellular phone by displayingwhite-colored characters on a black-colored background.

[0147] Additionally, the electroluminescent element according to theinvention can be applied to lighting equipment, wall of establishment,or the like which serves as a surface light source.

[0148] As mentioned above, an application range of the light-emittingdevice using the electroluminescent element according to the inventionis extremely wide. Further, the electroluminescent element according tothe invention can be easily controlled the color balance in white lightemission (white balance). Therefore, a well color balanced display canbe realized in electric appliances in various fields by applying theelectroluminescent element according to the invention thereto.

[0149] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdescribed, they should be construed as being included therein.

What is claimed is:
 1. An electroluminescent element comprising: anelectroluminescent layer formed between a pair of electrodes, whereinthe electroluminescent layer comprises at least a first light-emittinglayer and a second light-emitting layer, each of which has an emissionpeak in a wavelength region of 500 to 700 nm, and wherein the secondlight-emitting layer contains a phosphorescent material which formsexcimer at a concentration of 10 to 40 wt %.
 2. An electroluminescentelement comprising: an electroluminescent layer formed between a pair ofelectrodes, wherein the electroluminescent layer comprises at least afirst light-emitting layer and a second light-emitting layer, each ofwhich has an emission peak in a wavelength region of 500 to 700 nm, andwherein the second light-emitting layer contains a phosphorescentmaterial which forms excimer at a concentration of 10⁻⁴ to 10⁻³ mol/cm³.3. An electroluminescent element comprising: an electroluminescent layerformed between a pair of electrodes, wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of 500 to 700 nm, wherein the second light-emitting layercontains a phosphorescent material at a concentration of 10 to 40 wt %,and wherein a ratio of an emission peak intensity occurring in awavelength region of 500 to 550 nm in the second light-emitting layer toan emission peak intensity occurring in a wavelength region of 550 to700 nm in the second emitting layer is from 50 to 150%.
 4. Anelectroluminescent element comprising: an electroluminescent layerformed between a pair of electrodes, wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of 500 to 700 nm, wherein the second light-emitting layercontains a phosphorescent material at a concentration of 10 to 40 wt %,and wherein the electroluminescent element has a luminance of 100 to2000 cd/m².
 5. An electroluminescent element comprising: anelectroluminescent layer formed between a pair of electrodes, whereinthe electroluminescent layer comprises at least a first light-emittinglayer and a second light-emitting layer, each of which has an emissionpeak in a wavelength region of from 500 to 700 nm, wherein the secondlight-emitting layer contains a phosphorescent material at aconcentration of 10 to 40 wt %, and wherein a part of the phosphorescentmaterial exists at a certain distance from one another so that moleculesof the phosphorescent material can form excimer.
 6. Anelectroluminescent element comprising: an electroluminescent layerformed between a pair of electrodes, wherein the electroluminescentlayer comprises at least a first light-emitting layer and a secondlight-emitting layer, each of which has an emission peak in a wavelengthregion of 500 to 700 nm, wherein the second light-emitting layercontains phosphorescent materials comprising an organic metal complex ata concentration of 10 to 40 wt %, wherein central metals of thephosphorescent materials are a distance of 2 to 20 Å from one another.7. An electroluminescent element according to any one of claims 1 to 6,wherein the second light-emitting layer contains the phosphorescentmaterial at a concentration of 12.5 to 20 wt %.
 8. An electroluminescentelement according to claim 3, wherein the ratio of the emission peakintensity occurring in the wavelength region of 500 to 550 nm to theemission peak intensity occurring in the wavelength region of 550 to 700nm is from 70 to 130%.
 9. An electroluminescent element according toclaim 4, wherein the electroluminescent element has a luminance of 300to 1000 cd/m².
 10. An electroluminescent element according to any one ofclaims 1 to 6, wherein the second light-emitting layer has a thicknessof 20 to 50 nm.
 11. An electroluminescent element according to any oneof claims 1 to 6, wherein the second light-emitting layer has athickness of 25 to 40 nm.
 12. An electroluminescent element according toany one of claims 1 to 6, wherein the phosphorescent material has aplurality of emission peaks in a wavelength region of 500 to 700 nm; andany one of the plurality of emission peaks is excimer emission.
 13. Anelectroluminescent element according to any one of claims 1 to 6,wherein the first light-emitting layer has an emission spectrum with anemission peak in a wavelength region of 400 to 500 nm, wherein thesecond light-emitting layer has an emission spectrum with pluralemission peaks in a wavelength region of 500 to 700 nm, and wherein anyone of the plural emission peaks is excimer emission.
 14. Anelectroluminescent element according to any one of claims 1 to 6,wherein the phosphorescent material is an organometal complex withplatinum as a central metal.
 15. A light-emitting device having theelectroluminescent element according to any one of claims 1 to
 6. 16. Adisplay device having the electroluminescent element according to anyone of claims 1 to
 6. 17. A laptop computer having theelectroluminescent element according to any one of claims 1 to
 6. 18. Amobile computer having the electroluminescent element according to anyone of claims 1 to
 6. 19. An image reproduction device having theelectroluminescent element according to any one of claims 1 to
 6. 20. Agoggle type display having the electroluminescent element according toany one of claims 1 to
 6. 21. A video camera having theelectroluminescent element according to any one of claims 1 to
 6. 22. Acellular phone having the electroluminescent element according to anyone of claims 1 to 6.